The streets of Asmara are quiet now, according to reports from traders and deserting soldiers fleeing the besieged Ethiopian city.

Eritrean secessionist guerrillas have completed their encirclement of the provincial capital, and Ethiopian government troops occupying the northern city of 250,000 have become subdued and apprehensive in the expectation of a rebel assault.

A lack of fuel in Asmara has brought about a ban on all but military traffic. Electricity has been severely curtailed, and most factories and businesses have closed as a result.

Although nearly all economic activity in the city has ground to a halt, citizens of Asmara are able to move about freely for the first time in months.

Reports from persons crossing the lines to the rebel side say the troops holding Asmara appear listless and afraid, and they have ceased all acts of repression against the population.

Eritrean rebels give two reasons for this turn of events.

They feel Ethiopian forces were demoralized by the recent surprise announcement that the two main Eritrean rebel groups, which battled each other as well as government troops during the 16-year independence struggle, had formed a united front.

Eritrean leaders also believe their ambush of a government supply convoy last month on the road from the port city of Massawa to Asmara was a serious blow to morale. By cutting this road, which they claim was the last land corridor still open to the besieged city, Eritrean rebels say they left Ethiopia no way to resupply and reinforce its garrison except by air.

PREPARATIONS FOR the ambush, which took place about 18 miles west of Massawa where the road to Asmara runs through a narrow pass, had been under way for months.

Leaders of the Eritrean People's Liberation Front said they waited until the onset of the rainy season to spring the trap, so that the cloud cover would preclude Ethiopian air strikes on the rebel positions.

When a large convoy came down the road headed for Asmara, the rebels closed off both ends of the pass and systematically annihilated the government troops escorting it.

Three Soviet-supplied armed cars were reported captured, and six armored cars and tanks were destroyed in the three days of heavy fighting.

A contingent of Ethiopia's 30th Infantry Battalion accompanying the convoy was also destroyed, according to the Front, whose reports have generally been accurate.

The rebels said they captured 10 fully loaded fuel trucks, 20 trailer trucks and six army trucks scarrying equipment and food supplies. Many other trucks were reported to have burned in the fighting.

The real significance of this battle lies not in the captured equipment and supplies, but in the fact that the Eritrean rebels now control the road to Asmara.

Since Ethopian forces have been unable in the past two months to break through rebel lines on the western and southern outskirts of Asmara, the cutting of the Massawa road completes the isolation provincial capital.

If the rebels can keep the Massawa road closed, Asmara will have to be wholly supplied by air. This poses a logistical problem that the Ethiopian government - already strained to the breaking point by its war with Somali forces in the southeastern part of the country - will find difficult if not impossible to handle.

Because of the significance of the Massawa road, Eritrean rebels say they expect a major new Ethiopian effort to reopen this corridor. They vow, however, that not one more Ethiopian truck will ever reach Asmara.

ONE OF THE MAJOR problems that has long plagued the Eritrean independence movement is the existance of two major liberation fronts - the People's Front and its rival, the Eritrean Liberation Front.

Hence, the announcement that the two rebel groups were finally forming a united front set off major celebrations in areas of Eritrea under the control of the rebel forces.

In the city of Decamare, thousands of took to the streets to cheer the news, and the singing and dancing continued well into the night.

"They say they have different programs and plans for the future," one resident of Decamare said, "but they are our children, and we want them to stand together to kick out the enemy."

Just how long they will remain together, however, remains an open question.

While open fighting between the two groups officially ended in 1974, the two groups had been unable to reach any kind of alliance until now - and shoot-outs between rival forces in the field have left at least six dead in the past two months.

Rebel leaders suggest that the sudden Eritrean Liberation Front interest in joining a united front may well have a lot to do with the growing strength of the People's Front position.

The Eritrean Front which is estimated to have between 4,000 and 7,000 guerrillas, has suffered several humiliating defeats at the hands of government troops in recent months.

The People's Front on the other hand, signed up thousands of new recruits following a series of victories against government forces in the spring and summer. It now appear to have between 25,000 and 30,000 men under arms.

Organized in brigade strength and operating a sizeable fleet of captured vehicles, including tanks and armored personnel carriers, the People's Front has evolved from a ragtag guerrilla force into a highly disciplined, mechanized army engaging the government in conventional, set-piece warfare.

Sundanese President Jaafar Nimeri is credited with playing a decisive role in bringing about the agreement to frorm a united front. In the final analysis, however, the agreement was a simple recognition by both parties of the realities of the situation.

With government resistance collapsing, the two groups were running out of time if they were to avoid an Angolan-type situation in Eritrea.